The present invention relates to a dark level restoring circuit for recovering a dark level in a television receiver.
A video signal is generally transmitted in a state where a pedestal level used as a reference level for the darkest level is shifted toward a white level side. A difference between the pedestal level and a dark level is called a set-up level. The set-up level is not always equal and depends on the type of broadcasting station, TV camera, home video tape recorder (VTR), and the like. For this reason, the dark level must be precisely recovered in a television (TV) receiver (i.e., the dark level of the video signal must coincide with a cut-off level of a cathode-ray tube (CRT)) used in the receiver.
A method for decreasing a DC transmission factor and a method for decreasing a luminance level by an amount corresponding to the set-up level are conveniently used as a dark level restoring method. In the former method, since the set-up level (DC component) is decreased, no dark image is omitted. However, in a video signal having a low set-up level, the luminance level is shifted toward the dark level, so that a resolution of the dark image is deteriorated. In combination with this former method, a luminance ABL (automatic beam limiter) is usually used for controlling a beam intensity of the CRT by decreasing the luminance level for an image which requires too high a beam current. In this combination, however, deterioration of dark resolution undesirably occurs and precise dark restoration cannot be expected.
On the other hand, in the latter method, a dark peak level during one vertical scanning period in a video signal, for example, as shown in FIG. 1A, is detected and is controlled to coincide with the darkest level (pedestal level), as shown in FIG. 1B. This method is called a "dynamic picture system". A contrast ABL (or picture ABL) for controlling a beam intensity by decreasing a contrast of an image which requires a high beam current level of a CRT can be used in combination with this method, so that a stable dark level restoration is expected.
However, in this latter method, a luminance signal level always fluctuates due to a luminance level control corresponding to the dark peak level of the video signal. Therefore, a level ratio Y/C of a luminance signal Y to a chroma signal C is undesirably changed. For this reason, the density of a color is changed corresponding to the dark peak level control. For example, in a flesh tone of a human body (a luminance level of 50 to 80% with respect to a white peak level), when the dark peak level is shifted toward the white peak level, the luminance signal is decreased by the control loop. As a result, the chroma signal C changes to an undesirably high level with respect to the luminance signal Y, and flesh tones become too dense. Alternatively, when the dark peak level is low, the flesh tones become too light.
Furthermore, it is assumed that dark restoration is performed at time t.sub.1 by the control loop, as shown in FIG. 2A, and a signal P at the dark level side (e.g., a dark framed telop signal) is inserted at time t.sub.2, as shown in FIG. 2B. In this case, dark restoration is performed such that the dark peak level of the signal P coincides with the pedestal level by the control loop. For this reason, the brightness of an image is abruptly increased as shown in FIG. 2B. However, as shown in FIG. 2C, when the signal P has abruptly disappeared at time t.sub.3, the luminance level of the image is abruptly decreased in correspondence thereto. Therefore, the image becomes unsatisfactory and uncomfortable to watch.